The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Vehicles include a powertrain that generates drive torque used to propel the vehicle. Typically, the powertrain includes an internal combustion engine. Reciprocating-type engines produce torque by combusting an air and fuel (A/F) mixture or fuel-air charge in cylinders to drive pistons between a top-dead-center (TDC) position and a bottom-dead-center (BDC) position. Reciprocation of the pistons between the TDC and BDC positions drives rotation of a crankshaft, which produces the drive torque. At the TDC position, a volume of the combustion chamber is at its smallest volume. At the BDC position, the volume of the combustion chamber is at its largest volume.
Combustion occurs in combustion chambers defined by the cylinders and the pistons. In spark-ignition (SI) engines, combustion is initiated by an electrical discharge or spark that supplies energy to the fuel-air charge. Once initiated, combustion continues along a flame front for a period during which the piston is driven towards the BDC position. Generally, combustion timing is controlled and the flame front grows in a predictable and controlled manner and consumes nearly all of the fuel in the fuel-air charge. Combustion timing may be controlled by controlling spark timing, which may be specified relative to the TDC position. Spark advance generally refers to spark timing that occurs before the TDC position. Spark retard generally refers to spark timing that occurs after the TDC position.
Abnormal combustion is a term that is generally used to refer to a combustion process in which a flame front may be started by hot combustion chamber surfaces either prior to or after spark ignition, or a process in which a portion or all of the fuel-air charge may be consumed at abnormally high rates. Abnormal combustion phenomena include partial burning, misfire, knock, and pre-ignition. Knock is a term that is used to generally refer to an abnormal combustion process during which a portion of an end gas outside of the propagating flame front spontaneously ignites. Spontaneous ignition of the end gas, which includes the fuel, air, and residual gas mixture ahead of the propagating flame front, causes a rapid increase in local pressures and propagation of pressure waves of substantial amplitude across the combustion chamber. Knock is also used to refer to an audible noise that is generated by the pressure waves.
Pre-ignition is a term that generally refers to an abnormal combustion process in which the fuel-air charge is ignited by a source other than the electrical discharge provided by a spark plug. If the ignition occurs after the spark ignition, it is generally referred to as post-ignition. Following ignition, a turbulent flame develops at each ignition location and propagates in a manner similar to that occurring after spark ignition.
One type of pre-ignition is referred to as surface ignition, which generally refers to an abnormal combustion process in which a hot spot on the combustion chamber walls ignites the fuel-air charge prior to the arrival of the normal flame front. Hot spots may be created by an overheated valve or spark plug, combustion chamber deposit, or other energy source other than the spark. Surface ignition is often associated with what may be referred to as a “run away” phenomenon in which the pre-ignition occurs progressively earlier during successive engine cycles.
Another type of pre-ignition is referred to as stochastic pre-ignition. Stochastic pre-ignition may also be referred to as low-speed pre-ignition, since it generally occurs at relatively low engine speeds. Stochastic pre-ignition is generally associated with boosted engines such as turbocharged or supercharged engines. Stochastic pre-ignition typically occurs more randomly than other abnormal combustion processes and does not exhibit the typical run-away phenomenon characteristic of surface ignition.
Control systems have been developed to control combustion in an engine. In some conventional combustion control systems, spark timing is maintained below a spark limit or threshold to prevent an occurrence of knock and/or is retarded during corrective action to end or reduce an intensity of an occurrence of knock. However, these combustion control systems may not prevent or reduce an occurrence of pre-ignition.